Surface Hydration Inhibition Mechanism of Sodium Bentonite Clay of Polyamine Treatment Agents Under High Temperature

Abstract Na-bentonite is one of the most used water-based drilling mud additives due to its unique features for better well integrity. As a hydrophilic clay mineral, it is bound to hydration expansion altering drilling fluids’ properties and causing well instability. Small polyamine treatment agents are often used to avert the hydration of clay minerals, which can minimize the dangers of reservoir damage and wellbore instability. Revealing their inhibition mechanisms is vital for evaluating and designing inhibitor molecules by understanding their molecular behaviour at the sodium montmorillonite (Na-Mnt)–water interface. Herein, the inhibition mechanism of alkyl polyamines of different chain lengths, C5, C7, C12, and C18 was investigated using molecular dynamics (MD) simulation with a particular emphasis on the layering behaviour of alkyl chains, and effects of molecular chain length and temperature on adsorption and hydration inhibition ability. It was observed that the alkyl polyamine formed flat-lying conformation structures along the O-atoms while the amine groups were located mainly over the six-member rings, and the alkyl chains formed monolayers with carbon-chain lengths of C5, bilayers from C7 and C12, and trimolecular layers in C18. The amine groups interacted with the Mnt surface through H-bonds, electrostatic and van der Waals forces, forming organoclays thus reducing interlayer particle mobility as their mobility compared to alkyl chains was slightly less mobile. The distributions of polyamines reduced the amount of layer charge, and C7 gave better inhibition results at 2.553 wt.%. Furthermore, the inhibition stability of C7-diamine on the Mnt surface decreased with increasing temperature. A thorough investigation shows that increasing temperature can lead to desorption, which weakens the interaction between C7-diamine and Mnt, thus reducing adsorption, bonding and inhibition stability. The insights from our present study are beneficial for evaluating the inhibitory performances of organoclays and for the selection and molecular design of new bentonite inhibitors in drilling muds.